1. Field of the Invention:
This invention relates to eyepiece lenses and more particularly to eyepiece lenses suited for reducing the size of the finder.
2. Description of the Prior Art:
Three examples of the known two-component type eyepiece lens are depicted in FIGS. 1A to 1C. In general, eyepiece lenses of low magnifying power are constructed with one singlet and one cemented doublet. High performance eyepiece lenses of high magnifying power or high eyepoint distance, in which it is difficult to space the eye point away from the lens surface area are shown in FIGS. 1A to 1C. Such lenses are usually constructed with at least two components of different refractive power arrangement depending on the purpose. Generally, as shown in FIG. 1A, the two components are, from front to rear, a lens 1 of positive power and a cemented lens 2 of positive power. When high magnifying power is needed, the construction and arrangement of the two components is varied, as shown in FIG. 1B, to include a cemented lens 3 of positive power and a lens 4 of negative power. When a large eyepoint distance is wanted, the construction is reversed as shown in FIG. 1C, to include a lens 5 of negative power and a cemented lens 6 of positive power.
To employ such eyepiece lenses for observing the image of an object formed in optical instruments such as cameras, the finder image preferably looks erect vertically and horizontally. Cameras in which light from the photographic lens is directed to the finder by a quick return miror or a beam splitter such as a half-prism, including the so-called single lens reflex cameras, require an erecting optical system. Erecting optical systems include secondary imaging types using an erecting lens, and primary imaging types using a prism system and a mirror system, for example, a pentagonal roof prism, a Pechen prism, a porro-prism, or a porro-mirror. The pentagonal roof prism and Pechen prism, because they employ roof type reflection surfaces, tend to be expensive. In this respect, the use of a porro-prism or porro-mirror is advantageous. However, when the porror-mirror system is used, it is necessary to use at least four reflection surfaces in the optical path of the finder, and the length of the optical path thus has to be increased. Therefore, the use of the prior eyepiece lens in such finder optical systems results in the addition of the physical length of the eyepiece lens to the length of the optical path of the finder. This produces a disadvantageous increase in the bulk and size of the complete finder. Moreover, because the eyepiece lens is necessarily of increased focal length, its magnifying power is difficult to increase.